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Thread: DC coupled C.F. question

  1. #36
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    Quote Originally Posted by catalin gramada View Post
    I get a severe asymmetrical wave form at the follower output no matter by voltage applied.
    Not surprising that you would. This is very similar to a sloppy full wave rectifier circuit made from bipolars. The bipolar version was used in some guitar distortion pedals.

    Look at it this way: With a common cathode resistor, whichever grid is most positive raises the cathode voltage by letting current through. This makes the other side turn off more, letting the higher grid voltage "win" even more. As long as one half can supply enough current by itself, the cathode voltages will follow the highest grid.

    That means that the highest side wins. If this circuit is fed from a phase inverter, equal and opposite polarity signals, the signal out at the cathodes (and plates, if you join the plates into a common plate resistor) gets to be a full wave rectified version of the input signal, with some slop determined by matching of the two sides. Tubes will probably do a softer version due to the much smaller transconductance.

    It's not a good mixer circuit as is. If you split the cathode circuits, with a cathode biasing resistor per triode and take your output from a single common plate resistor, it gets to being a good mixer.

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  2. #37
    "Thermionic Apocalypse" -JT nickb's Avatar
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    Here's a suggestion for a mixer that has a gain of unity, has good isolation between inputs, excellent linearity, quite respectable power supply rejection, a low output impedance of about 1K ohm and which can easily be expanded to more than two inputs. You can tweak R4 to change the gain a little.

    Click image for larger version. 

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    Last edited by nickb; 01-14-2019 at 08:42 PM.
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    Thanks. For some reason I tried to omited mixer resistors. The channels have to mix are mixed directly from tone stacks, sort of Ampeg B15 arrangement. I tried from 220k to 1M. frequency compensated by tweaked series cap after joint point. Didn.t found any convenient formula from my sound taste perspective to keep it as each channel sound alone and loosed some gain aso. Finally I decided to keep 1M pair and a switch circuit which short one resistor or another. In this way keep one channel directly coupled from tone stack without to load it too much.

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    Last edited by catalin gramada; 01-14-2019 at 10:42 PM.
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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    Quote Originally Posted by nickb View Post
    Here's a suggestion for a mixer that has a gain of unity, has good isolation between inputs, excellent linearity, quite respectable power supply rejection, a low output impedance of about 1K ohm and which can easily be expanded to more than two inputs. You can tweak R4 to change the gain a little.

    Click image for larger version. 

Name:	Mixer.JPG 
Views:	45 
Size:	42.6 KB 
ID:	51937
    Nice tube example of a virtual ground mixer!

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  5. #40
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    Quote Originally Posted by catalin gramada View Post
    Thanks. For some reason I tried to omited mixer resistors. The channels have to mix are mixed directly from tone stacks, sort of Ampeg B15 arrangement. I tried from 220k to 1M. frequency compensated by series cap after joint point. Didn.t found any convenient formula from my sound taste perspective to keep it as one channel sound alone and loosed some gain aso. Finally I decided to keep 1M pair and a switch circuit which short one resistor or another. In this way keep one channel directly coupled from tone stack without to load it too much.
    Are you speaking of Nick's proposal? No good idea to omit input mixing resistors with this circuit, as the (current) summation point at the grid is very low impedance and will load down signal sources if directly connected.

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    Quote Originally Posted by Helmholtz View Post
    Are you speaking of Nick's proposal? No good idea to omit input mixing resistors with this circuit, as the (current) summation point at the grid is very low impedance and will load down signal sources if directly connected.
    No, I spoke about what I already done trying to keep the channels isolated as much. I will wire a tube as Nick suggest to see how it works on my project.

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    Quote Originally Posted by nickb View Post
    Here's a suggestion for a mixer that has a gain of unity, has good isolation between inputs, excellent linearity, quite respectable power supply rejection, a low output impedance of about 1K ohm and which can easily be expanded to more than two inputs. You can tweak R4 to change the gain a little.

    Click image for larger version. 

Name:	Mixer.JPG 
Views:	45 
Size:	42.6 KB 
ID:	51937
    Localized inverting feedback.... good channel isolation. A bit of a virtual earth stage, no?

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    A bit of a virtual earth stage, no?
    See post #39 (earth=ground)

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    Quote Originally Posted by Helmholtz View Post
    See post #39 (earth=ground)
    Right i hadn’t seen your post yet when I replied. Sorry for the redundancy.

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